1. Field of the Invention
The aspects of the disclosed embodiments relate to a system and a method for controlling an actuator, particularly a turbo compressor, and more specifically to an actuator comprising at least one turbo compressor, with an outlet in said at least one turbo compressor for conveying compressed gas to at least one cooler, and a shaft for rotating said at least one turbo compressor, for increasing the pressure of the gas to be supplied to the actuator. The actuator relates to a turbo-type compressor in which the blades of the turbo wheels rotate at a high speed and thereby increase the pressure of the gas. The pressure increase may take place at one or more stages.
2. Brief Description of Related Developments
Primarily, two types of compressors are known: positive displacement compressors and dynamic compressors. The positive displacement compressors include, for example, screw compressors, piston compressors, and vane compressors. The positive displacement compressors are also called static compressors. The dynamic compressors, in turn, can be divided into the centrifugal and axial flow types (radial compressors and axial compressors, respectively). In dynamic compressors, air is provided with high kinetic energy which is then converted to pressure. Dynamic compressors can also be called turbo compressors.
In known solutions for controlling turbo compressors, the quantity of air and the pressure increase produced by the turbo compressor are adjusted, among other things, by guide vanes installed at the air intake of the turbo compressor, by diffuser vanes installed downstream of the impeller wheel, by changing the rotating speed of the actuator, by recirculating some of the air from the pressure side back to the suction side, or by blowing some of the compressed air out of the process via a valve.
Known methods for adjusting turbo compressors relating to combustion engines and their turbo drives are presented in technical literature in the field, publications of turbo compressor manufacturers, as well as patent publications, for example Japanese patent application publication JP 2008286039 A as well as international patent application publication WO 1999049222 A1.
Some drawbacks of the known solutions for controlling turbo compressors include, for example, the fact that they have a narrower range of capacity control compared with, for example, the range of capacity control of screw compressors, as well as the fact that when applying them, the total efficiency of the process and the actuator is lower, because of e.g. fluctuation in the need of gas, compared with e.g. the efficiency of screw compressors.
The control solution disclosed in patent publication JP 2008286039 A is designed particularly for preventing stall of turbo compressors in combustion engines, not for producing oilless compressed air with turbo compressors.
In the invention disclosed in publication WO 1999/44222 A1, the solution for generating pressure by a turbine is implemented by integrating a turbine unit in one compressor unit, which corresponds to a turbocharger structure known as such. The other compressor units are provided with separate motor units of their own. These motor units may be either with or without a step-up gear. In the solution according to said publication, superfluous air/gas, in other words air/gas that is momentarily not needed in the process, is continuously led to the turbine to provide the shaft output needed by one compressor unit. Consequently, this solution does not make it possible to adjust the production of compressed air in a wide adjustment range without changing an operating point, such as the efficiency or the rotating speed, of the compressor units.